1. Field of the Invention
The present invention relates to means for measuring rotation rates. More particularly, this invention pertains to apparatus based upon Coriolis acceleration sensing for measuring rotation rates about three axes.
2. Description of the Prior Art
Numerous arrangements exist for measuring rotation rate about a preselected axis in inertial space. Such apparatus, commonly designated a gyroscope, forms an essential element of any inertial navigation system. Gyroscopes include, for example, complex and difficult-to-manufacture gimballed spinning rotors, strapdown sensors such as the ring laser and fiber optic gyroscope. All of the above-named rate sensing devices are characterized by complexity of manufacture, expensive maintenance, or both.
Navigation systems must measure space-dependent variables, such as rotation, with respect to (or about) a set of three body fixed orthogonal axes. The design of a three-axis rotation sensor that is sufficiently compact, economical and realizable in a manufacturing sense is beset by numerous difficulties. Among these is the conventional usage of multiple "drive mechanisms" or their analogs in triaxial rotation sensors regardless of operating principle. For example, in a ring laser or a fiber optic gyroscope, a single lasing cavity is devoted to each input axis and three lasing cavities are therefore required in a triaxial sensor of either of such types. Such a device is shown, for example, in U.S. Pat. No. 4,795,258 of Martin entitled "Nonplanar Three-Axis Ring Laser Gyro With Shared Mirror Faces," property of the Assignee herein. Spinning wheel gyros are limited to two axes of measurement, necessitating the use of an additional drive mechanism for the third input axis.
A system for measuring rotation rates (and accelerations) with respect to a set of three orthogonal axes that requires only a single drive mechanism is taught in a series of United States patents of Stewart and Fersht (U.S. Pat. Nos. 4,996,877 entitled "Three Axis Inertial Measurement Unit With Counterbalanced Mechanical Oscillator; U.S. Pat. No., 5,007,289 entitled "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator; and U.S. Pat. No. 5,065,627 entitled "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator.") The system disclosed in each of those patents employs a drive mechanism comprising a plurality of vanes having piezoelectric strips mounted thereon for affecting deflections that cause a pair of counterbalanced platforms to oscillate out of phase about a common axis within a housing or case. Closed loop accelerometers are mounted about the peripheries of the platforms and oriented at predetermined angles with respect to the axis of oscillation. The angular orientations permit the accelerometers to determine angular rates about the pendulous axes of the three accelerometers through measurement of the Coriolis accelerations (and linear accelerations along the three input axes.)
While achieving the desirable objective of accomplishing measurements about three axes by means of a single instrument drive, the device disclosed is limited in part by the use of closed loop accelerometers. The unavoidable presence of so-called squeeze film damping in closed loop (silicon) accelerometers necessarily limits their bandwidth to 1000 Hz or less. As a result, while such devices adequately measure linear accelerations, their accuracies are limited in regard to the measurement of the higher frequency Coriolis accelerations. Ideally, the bandwidth of the accelerometers for this purpose should be four (4) or five (5) times that of the driven frequency of about 900 Hz. Thus the accuracy of closed-loop silicon accelerometers for Coriolis measurement in a rotation sensor is limited.
Furthermore, in the patented device, the oscillating structure is mechanically linked to its case or enclosure. As a result, the oscillating platforms can react to the case, thereby dissipating the energy transmitted by the piezoelectric-driven vanes and thereby subjecting the instrument to mounting sensitivities.